The research developed in this PhD thesis Préstamos del catalán en el léxico español (Lexical Borrowings from Catalan into Spanish) is a diachronic study of lexical borrowings between two Romance languages in a direction which has been rarely studied: words from Catalan integrated throughout history into Spanish or into any of his dialects (especially Aragonese and Murcian).The introduction of this research approaches theoretical and conceptual questions about the concept of préstamo and about other concepts related to the process of lexical transference. It also includes a summary of research on the relationship between Spanish and Catalan from different points of view (especially historical, political, cultural and, consequently, linguistic perspectives).The elaboration of the corpus is explained next. This corpus has been compiled from a selection of the etymological content of two lexicographical sources of information. The principal source is a specialized dictionary: Diccionario crítico etimológico castellano e hispánico (DCECH), by J. Corominas in collaboration with J. A. Pascual. The number of borrowings from Catalan obtained has been contrasted and completed with the information found in a general dictionary, the latest edition of the Diccionario de la Lengua Española (DRAE), by the Real Academia Española.The central part of this investigation consists of a practical study of the consequences of the Catalan influence into the Spanish vocabulary. It analyses the function of Catalan in the process of lexical transference. Special attention is payed to the study of the criteria which are more often used in the DCECH for the identification of borrowings and, specifically, for the defense of the hypotheses of catalanism: internal or linguistic arguments (phonetics, derivative morphology and semantics) and external or extralinguistic arguments (chronology-documentation, use and linguistic geography). The analysis of this material has been compared with the data and results found in monographs and other relevant sources of information. This study shows that individual justification (the use of only one argument) is not enough for the defense of a particular type of transference, so the combination of criteria of different kinds is preferable. The most important reason rests on the (basically phonetic) similarities shared by Romance languages and therefore on the difficulties that exist at the moment of identifying which one of them (if Catalan more than Occitan or French, for example) has been acted as intermediary for the Spanish language. The specific analysis of the corpus is ended with a reference to the etymological information of the DRAE, a general monolingual dictionary. The contrast of data between the two lexicographical sources of information shows a substantial coincidence in the consideration of catalanism, although it is also possible to observe discrepancies, aspect which has to be valued with the opinion of other bibliographical sources in order to accept the most plausible hypothesis.The results which have been achieved in this research demonstrate that the vocabulary of a language is a faithful testimony of interlinguistic relationships. The criteria for the justification of the etymological hypotheses show the difficulties to determine the direction of borrowings among Romance languages. The integration of a diversity of arguments contributes, in that case, to consolidate and guarantee the Catalan influence into the Spanish language throughout history.